"By all external appearances, the walls of the dome on the U.S. Capitol building are a solid marble edifice. But the shell is actually hollow and supported like an iron-framed skyscraper. The tight space between the exterior facade and the interior walls contains a stairwell with 365 steps.
At the top of the dome the stairs lead to a small room known as the Tholus. The government has released several photos over the years from the Tholus looking out, but there’s only one mysterious photo floating around (from Wikimedia, of all places) that shows the interior of the Tholus.
The familiar present-day structure is actually the Capitol building’s second dome. The original so-called Bulfinch Dome was smaller, made of wood and clad with copper."